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LIBRARY 
STATE P£\4$T9B£>ARD E _ 841 

United States Department of Agriculture 

Agricultural Research Administration 

Bureau of Entomology and Plant Quarantine 

THE LARCH SAWFLY EPIDEMIC IN THE LAKE STATES 
Surveys and Spraying Experiments in 1949 and 1950^ 

9/ 
By James W. Butcher-^ and Charles B. Eaton, 

Division of Forest Insect Investigations 



The larch sawfly (P ristiphora erichsonii (Htg.)), a well-known enemy 
of tamarack (Larix laricina (Du Roi) K. Koch), has once again appeared 
in epidemic numbers in the Lake States. Since the destructive outbreaks 
of the 19th and early part of the 20th century, during which the insect is 
reputed to have spread westward from the eastern seaboard to British 
Columbia and the Yukon (Swaine 5), this sawfly has remained innocuous 
in the young stands that have developed in the wake of these outbreaks. 
Its resurgence in the present epidemic apparently began in northern 
Minnesota as early as 194C (Hodson 2). The pattern of build-up in the 
extensive tamarack stands in the State between 1940 and 1947 has been 
charted by Hodson and Christensen (3). 

In 1949 and 1950 investigations of the larch sawfly were undertaken 
by the Milwaukee, Wis., laboratory of the Bureau of Entomology and 
Plant Quarantine. This action was prompted by the inadequacy*- of existing 
information on the status of the outbreak, the biology of the insect, and 
the potentialities for destruction in the stands under attack. The objectives 
were (1) to conduct periodic surveys of the infestation, (2) to study the 
biology and ecology of the insect, and (3) to evaluate its susceptibility to 
control with recently developed insecticides. This paper reports tech- 
niques and results of the surveys and control experiments carried out 
during these years. 



1/ This work was conducted in cooperation with the Division of Ento- 
mology and Economic Zoology, University of Minnesota. Facilities and 
assistance were supplied by the U. S. Forest Service; the Minnesota 
Department of Conservation and Department of Agriculture, Dairy, and 
Food; and the Office of the St. Louis County (Minnesota) Land Commis- 
sioner. 

2/ Now with Office of the State Entomologist, Minnesota Department 
of Agriculture, Dairy, and Food. 



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Aerial Reconnaissance 



The survey work carried out in 1949 was necessarily less thorough 
than was desirable. Limitations in personnel and time, together with the 
size of the area to be covered (about 15 million acres in Minnesota alone), 
made it impossible to survey infested stands adequately from the ground. ' > 

Fortunately, however, exploratory airplane flights disclosed the feasibility 
of delimiting defoliated stands from the air and, despite its obvious short- 
comings, this technique became the chief means of defoliation detection. ' 

In 1949 an observer and a mapper accompanied the pilot on most of 
the survey flights. At an air speed of 70 miles per hour and an altitude 
of 1, 000 feet, it was possible to detect the more obviously defoliated 
areas in a strip up to 8 miles wide (4 miles on each side of the plane), 
and to plot them on a map at a scale of 1 inch = 1 mile. It is calcu- 
lated that the area observed during the 12 hours' flying time devoted 
to this work was about 4,300,800 acres. The information obtained 
was checked against aerial photographs of forest type supplied by the 
United States Forest Service and the Office of the St. Louis Countv 
(Minnesota) Land Commissioner. Because of the concentric pattern of 
defoliation, particularly in the larger bogs with extremely wet interiors, 
it was not possible to convert the survey data into graduated infestation 
levels. However, the method did permit the spotting of centers of heavy 
defoliation and provided a basis for the construction of a map illustrating 
the apparent limits of the 1949 infestation. Visible defoliation was clas- 
sified as follows: Light, less than 25 percent; medium, 26 through 50 
percent; and heavy, 51 through 100 percent. 

The conduct of aerial reconnaissance surveys in 1950 differed little 
from the method used in 1949. However, since there was more time for 
ground surveys, it was possible to forego detailed mapping from the air. 
Because the chief object was to confirm the pattern of the outbreak as 
constructed from plot records, flights were carried out at an altitude 
of 2,000 feet to increase the area of coverage. In this manner at least 
6,567,680 acres were examined for evidence of defoliated tamarack. 

As there was no appreciable extension in 1950 of the area of heavy 

1949 defoliation, it is possible to present the results of the 1949 and 

1950 surveys in one map (fig. 1). The only apparent difference in con- 
ditions for the 2 years occurred in the north-central section of Min- 
nesota (zone 4), where a marked decline in defoliation was observed in 
1950. This section embraces most of the tamarack within the Chippewa 
National Forest. The recession is attributable both to excessive rainfall 
during the period of cocooning in the summer of 1949, when large num- 
bers of mature and newly cocooned larvae were drowned, and to the 



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prolonged immersion of overwintered prepupae^r as a consequence of 
extremely high water levels in most of the larger bogs in this area during 
the spring of 1950. One or both of these conditions drastically reduced 
the population of sawfly larvae on the Chippewa National Forest and its 
environs during this period. 

Ground Reconnaissance 

In 1950 a modified roadside-sampling technique was devised, and the 
location of each observation point was recorded as the speedometer dis- 
tance from a prominent check point along well-marked roads. A total of 
36 plots were established, of which 23 were located in Minnesota, chiefly 
on the Superior and Chippewa National Forests, 4 in northern Wisconsin, 
and 9 in Michigan (fig. 1). Data on tree mortality and degree of defolia- 
tion, based on examination of at least 10 randomly selected trees per 
plot, were obtained together with routine observations on stand and site 
characteristics. These plots and records will form the basis for sub- 
sequent surveys as long as the outbreak coutinues. 

In 1949 and 1950 the outbreak was confined to 21 northern Minnesota 
counties, but in only 9 of these counties was defoliation in excess of 50 
percent during either year. In the remaining counties defoliation averaged 
less than 15 percent. Evidence of sawfly work was seen in only one of the 
plots in Wisconsin (near Brule), and not at all in Michigan. Damage was 
observed on individual trees at Park Falls and Griffith State Nursery in 
Wisconsin, and at Merriweather in Michigan, but nowhere in these two 
States was the defoliation general. 

Infestation in Minnesota 

The State of Minnesota possesses almost two-thirds of the total area 
of tamarack type^ in the Lake States (Cunningham 1). Most of the area 
is in the northern tier of counties that lie within the present limits of 
sawfly infestation. Table 1 shows a breakdown by counties of the tamarack 
acreage within each defoliation zone (see fig. 1). The acreage of tamarack 
type for each county was supplied by the Lake States Forest Experiment 
Station. The sources of these estimates are the Forest Surveys for 1938 
and surveys in progress. Figures for the Superior National Forest are 
based on the forest-management plan for 1936, and data for St. Louis 
and Lake Counties outside the National Forest boundaries on estimates 
provided by members of the Superior National Forest staff. 



3/ Butcher, J. W. Studies on larch sawfly populations, factors con- 
tributing to their fluctuations and early recognition of host decadence. 
1951. (Unpublished thesis University of Minnesota.) 

4/ Stands containing 50 percent of swamp conifers with tamarack 
outweighing other species. 



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Table 1. --Estimated acreages of tamarack type infested with the larch 
sawfly in each defoliation zone (see fig. 1) in northern Minnesota 



County 


Zone 1 


Zone 2 


Zone 3 


Zone 4 


Total 


Lake of the Woods 


62,000 


_ , 


mm 


— 


62,000 


Koochiching 


112,480 


35,520 


- 


- 


148,000 


Clearwater 


- 


- 


- 


8,400 


8,400 


Beltrami 


26,404 


25,760 


- 


12,236 


64,400 


Hubbard 


- 


- 


3,900 


- 


3,900 


Wadena 


- 


- 


1,500 


- 


1,500 


Cass 


- 


14,514 


14,514 


10,072 


39,100 


Crow Wing 


- 


- 


8,528 


1,872 


10,400 


Aitkin 


- 


- 


16,154 


23,246 


39,400 


Itasca 


- 


23,932 


- 


14,668 


38,600 


Carlton 


- 


- 


350 


3,150 


3,500 


Lake and St. Louis 


27,812 


- 


- 


11,919 


39,731 


Cook 


89 


- 


- 


- 


89 


Total 


228,785 


99,726 


44,946 


85,563 


459,020 



Control Experiments 



In most cases where successful control of forest-insect populations 
has been achieved through the use of chemicals, there have not been the 
complicating biological limitations to insecticide effectiveness that have 
been noted for the larch sawfly. One objective of the control experiments 
was to try to assess the importance of life-history traits that might enable 
some of the sawflies to escape contact with the insecticide unless more 
than one application was made. Quite apart from the separate effects of 
individual influences, however, it seemed desirable to determine the 
degree of control that could be achieved under prevailing conditions. 

Two main obstacles to high sawfly mortality from insecticides were 
apparent before the control experiments were started. One was the 
tendency for some of the overwintering population (prepupae) to remain 
in the soil for more than a year. The other was the relatively long period 
when sawfly adults were active, so that susceptible and nonsusceptible 
life stages overlapped, and at no time was the entire population vulner- 
able. The larval and adult stages were susceptible to sprays, but the 
egg and cocoon stages were not. 

In 1949 aerial spraying with technical DDT in fuel oil, at the rate of 
1 pound per gallon per acre, was carried out on 80 acres of lightly in- 
fested tamarack on the Chippewa National Forest when most of the larvae 
were in the late instars. From the time of application until feeding 



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terminated, 81 percent more frass was produced in an adjacent check 
plot than in the treated plot. 

In 1950 it was decided to explore the possibility of securing better 
control with the same method by treating as near as possible to the peak 
of adult activity as well as of frass production by the larvae. For 
spraying the adults two areas known to be heavily infested in 1949 were 
selected- -one near Ebro, Minn., containing 90 acres, and the other 
near Bagley, Minn., containing 80 acres. The areas were treated on 
June 30, the date coinciding with the best available estimates of the 
actual peak of adult emergence. 

Each week throughout the season frass was collected in trays, 3 by 
1 by 1/2 feet, set out in two sprayed and two check plots, one tray in 
each plot. The total collection for the season averaged 25.36+4.18 ml. 
from the sprayed plots and 38.62+4.44 ml. from the check plots. These 
figures indicate a 34.3-percent reduction in frass in sprayed plots on the 
assumption that both check and sprayed areas had the same populations 
before spraying. The differences between sprayed and unsprayed plots 
subsequent to spraying, while statistically significant, were so small 
that no recommendations for adult spraying seemed advisable. 

In 1950 a treatment to control the larvae was made in two blocks of 
heavily infested tamarack at Ely, Minn. Again the insecticide was DDT 
applied at the rate of 1 pound per gallon of fuel oil per acre. The appli- 
cation was made on August 2, approximately a week before the peak of 
larval feeding, although between 10 and 20 percent of the season's pop- 
ulation had cocooned by this time. The effect of the treatment was 
evaluated in the same manner as for the adult spraying, except that 
the trays were installed 3 days before treatment in order to check the 
uniformity of the sawfly population on the plots. The average volumes 
of frass per tray collected weekly for the check and sprayed plots are 
given below: 

Milliliters of frass 
Date Sprayed plots Check plots 

August 2 

9 

16 

23 

Up to the time of spraying, there was no statistically significant 
difference between frass volume from the check areas and that from 
the sprayed areas, which could be attributed to anything but sampling 
variation (Snedecor 4). One week later, however, there were signifi- 
cant differences, indicating two populations with unequal means, the 
differences due conceivably to spraying. Differences between the 



6.8 


9.3 


20.5 


82.1 


3.8 


35.3 


0.1 


5.0 



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seasonal frass accumulations were also significant and, assuming com- 
parable populations in check and sprayed plots prior to treatment, pop- 
ulation reduction based on frass-production figures was 77 percent in the 
sprayed plots. 

More work needs to be done to verify these results. However, the 
population reduction given above does seem consistent with the expected 
mortality, considering the obstacles to control that stem from the spread 
of life-history stages. This work has established the susceptibility of 
larch sawfly larvae to DDT and, within limits imposed by the circumstances 
under which the work was done, demonstrates the degree of control that 
could be expected under conditions prevailing in 1950. 

Conclusion 



In view of the results of these tests, the usefulness of DDT sprays to 
control larch sawfly outbreaks is open to question. If allowance is made 
for the 10 to 20 percent of the total population that cocooned, and for the 
fact that a reduction of only 77 percent of the larval population in the 
sprayed plots can be accounted for through reduced frass production, it 
is evident that mortality of the total resident population from a single 
spray application may not exceed 70 percent. The 30 percent surviving 
may be enough to perpetuate the outbreak. However, any conclusions on 
population reduction drawn from these experiments must take into con- 
sideration the high mortality that occurs from climatic and biological 
factors. The exact effect of these influences is not known, but it is 
believed that unfavorable moisture and temperature conditions cause 
mortality among overwintering prepupae. Sawfly survival is also in- 
fluenced somewhat by parasites, and to a greater extent by rodents. 

Although its permanent effect on the sawfly population may be of 
questionable value, spraying may be useful for minimizing damage to 
stands already weakened by repeated attacks. Apart from long-range 
sawfly control, protection from imminent defoliation could mean the 
difference between death or survival of a particular stand. In this 
connection knowledge of the susceptibility of larvae accessible to the 
insecticide is of value. The possible effect of a 70 -percent reduction in 
the larval population on the succeeding generations cannot be estimated, 
but observed differences between sprayed and unsprayed plots at Ely, 
Minn., the year after treatment indicate that such differences might be 
considerable under some conditions. Thus, even a partial reduction in 
sawfly populations might offer protection from severe defoliation for 
more than one season. 



UNIVERSITY OF FLORIDA 



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3 1262 09239 6414 

Literature Cited 

(1) Cunningham, R. N. 

1950. Forest resources of the Lake States region. U. S. Forest 
Serv., Forest Res. Rpt. No. 1, 57 pp. 

(2) Hodson, A. C. 

1940. Minnesota forest insect survey for 1940. Minn. Agr. 
Expt. Sta., Misc. Jour. Ser., Paper No. 448. 

(3) and Christensen, C. M. 

1941-47. Minnesota forest insect and disease surveys. Minn. 
Agr. Expt. Sta., Sci. Jour. Ser., Paper Nos. 1997 
(1941), 2084(1942), 2194(1943), 2296 ( 1945); Misc. 
Jour. Ser., Paper No. 634 (1947). 

(4) Snedecor, George W. 

1946. Statistical methods. 485 pp. Ames, Iowa. 

(5) Swaine, J. M. 

1933. The relation of insect activities to forest development as 
exemplified in the forests of eastern North America. 
Sci. Agr. 14(1): 8-31.